ACCELERATED INTERLEAFLET TRANSPORT OF PHOSPHATIDYLCHOLINE MOLECULES IN MEMBRANES UNDER DEFORMATION

Biological membranes are lamellar structures composed of two leaflets capable of supporting different mechanical stresses, Stress difference s between leaflets were generated during micromechanical experiments i n which long thin tubes of lipid (tethers) were formed from the surfac es of giant phospholipid vesicles. A recent dynamic analysis of this e xperiment predicts the relaxation of local differences in leaflet stre ss by lateral slip between the leaflets. Differential stress may also relax by interleaflet transport of lipid molecules (''flip-flop''). In this report, we extend the former analysis to include interleaflet li pid transport. We show that transmembrane lipid flux will evidence its elf as a linear increase in tether length with time after a step reduc tion in membrane tension, Multiple measurements were performed on 24 d ifferent vesicles composed of stearoyl-oleoyl-phosphalidylcholine plus 3% dinitrophenol-linked di-oleoyl-phosphatidylethanolamine. These tet hers all exhibited a linear phase of growth with a mean value of the r ate of interlayer permeation, c(p) = 0.009 s(-1). This corresponds to a half-time of similar to 8 min for mechanically driven interleaflet t ransport. This value is found to be consistent with longer times obtai ned for chemically driven transport if the lipids cross the membrane v ia transient, localized defects in the bilayer.